This invention is in the field of semiconductor integrated circuits. Embodiments of this invention are more specifically directed to capacitor structures that define analog voltages in such integrated circuits.
An important type of semiconductor integrated circuits are circuits that implement analog circuit functions, in which input and output signals and information are communicated and processed in the analog domain. Typically, analog integrated circuit functions rely on reference levels (voltages and currents) that are established and regulated on-chip. Proper functioning of the analog integrated circuit, and particularly such functioning over variations in power supply voltage, temperature, and other operating conditions, often critically depends on the stability of reference voltages and currents over such variations. However, manufacturing variations reflected in physical parameters of the integrated circuits can affect the reference levels as generated in these integrated circuits. Accordingly, many analog integrated circuits include some ability to “trim” or adjust the on-chip precision reference circuits, as well as other circuit functions within those integrated circuits. Trimming is typically performed at manufacture, after electrical measurement or other evaluation of the performance of the raw circuit as manufactured.
Recently, programmable non-volatile memory elements have been considered for use as trimming elements, for example in replacement of fuses or antifuses. Examples of these non-volatile memory elements include floating-gate metal-oxide-semiconductor (MOS) transistors, in which the state of the transistor is defined by charge trapped at a floating gate electrode, such as a floating capacitor plate. Programming of the device is accomplished through such mechanisms as Fowler-Nordheim tunneling, and hot carrier injection. Programming of floating-gate structures is attractive as a trimming technique because of the precision with which charge may be programmed according to modern programming methods, and also because the programming operation can be carried out by purely electrical means.
However, trimming of circuit parameters by way of floating-gate elements requires retention of the trapped charge at the floating gate for the life of the device, considering that the trimming may only be performed at the time of manufacture. Conventional capacitor dielectric films in analog integrated circuits have been observed, in connection with this invention, to exhibit some degree of leakage over time. An example of such a conventional capacitor dielectric is silicon nitride deposited by plasma-enhanced chemical vapor deposition (PECVD). As such, the use of floating-gate capacitor technology in conventional analog circuits would require additional costly processes such as deposition of dielectric films specifically for the programmable capacitors, deposition and patterning of an additional conductor layer, and the like.
Ahuja et al., “A Very High Precision 500-nA CMOS Floating-Gate Analog Voltage Reference”, J. Solid-State Circ., Vol. 40, No. 12 (IEEE, December 2005), pp. 2364-72 describes the use of floating-gate technology in precision analog reference circuits. In this article, the floating-gate device is constructed as a double-level polysilicon device. Tunneling regions between the two polysilicon levels is formed as a 400 Å film of silicon dioxide. It is believed that the manufacturing process implemented into this structure is relatively costly, given the requirement that a separate tunneling oxide film be deposited. In addition, this approach uses a relatively thick tunneling oxide film, which results in a relatively small capacitance per unit area.
Copending and commonly assigned U.S. application Ser. No. 13/070,222, filed Mar. 23, 2011, entitled “Low Leakage Capacitor for Analog Floating-Gate Integrated Circuits”, and incorporated herein by reference, describes an analog floating-gate electrode in an integrated circuit. As described therein, the analog floating-gate electrode is formed as a unitary polycrystalline silicon gate element with portions serving as a transistor gate electrode, a plate of a metal-to-poly storage capacitor, and plates of poly-to-active tunneling capacitors. Silicide-block silicon dioxide blocks the formation of silicide cladding on the electrode, while other polysilicon structures in the integrated circuit are silicide-clad.